3 files changed, 52 insertions, 83 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 1421b02a7905..a8cb00c0c6d9 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -913,15 +913,21 @@ static inline bool tcp_in_initial_slowstart(const struct tcp_sock *tp)
 	return tp->snd_ssthresh >= TCP_INFINITE_SSTHRESH;
 }
 
+static inline bool tcp_in_cwnd_reduction(const struct sock *sk)
+{
+	return (TCPF_CA_CWR | TCPF_CA_Recovery) &
+	       (1 << inet_csk(sk)->icsk_ca_state);
+}
+
 /* If cwnd > ssthresh, we may raise ssthresh to be half-way to cwnd.
- * The exception is rate halving phase, when cwnd is decreasing towards
+ * The exception is cwnd reduction phase, when cwnd is decreasing towards
  * ssthresh.
  */
 static inline __u32 tcp_current_ssthresh(const struct sock *sk)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 
-	if ((1 << inet_csk(sk)->icsk_ca_state) & (TCPF_CA_CWR | TCPF_CA_Recovery))
+	if (tcp_in_cwnd_reduction(sk))
 		return tp->snd_ssthresh;
 	else
 		return max(tp->snd_ssthresh,
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 38589e464e63..e2bec815ff23 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2470,35 +2470,6 @@ static inline void tcp_moderate_cwnd(struct tcp_sock *tp)
 	tp->snd_cwnd_stamp = tcp_time_stamp;
 }
 
-/* Lower bound on congestion window is slow start threshold
- * unless congestion avoidance choice decides to overide it.
- */
-static inline u32 tcp_cwnd_min(const struct sock *sk)
-{
-	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
-
-	return ca_ops->min_cwnd ? ca_ops->min_cwnd(sk) : tcp_sk(sk)->snd_ssthresh;
-}
-
-/* Decrease cwnd each second ack. */
-static void tcp_cwnd_down(struct sock *sk, int flag)
-{
-	struct tcp_sock *tp = tcp_sk(sk);
-	int decr = tp->snd_cwnd_cnt + 1;
-
-	if ((flag & (FLAG_ANY_PROGRESS | FLAG_DSACKING_ACK)) ||
-	    (tcp_is_reno(tp) && !(flag & FLAG_NOT_DUP))) {
-		tp->snd_cwnd_cnt = decr & 1;
-		decr >>= 1;
-
-		if (decr && tp->snd_cwnd > tcp_cwnd_min(sk))
-			tp->snd_cwnd -= decr;
-
-		tp->snd_cwnd = min(tp->snd_cwnd, tcp_packets_in_flight(tp) + 1);
-		tp->snd_cwnd_stamp = tcp_time_stamp;
-	}
-}
-
 /* Nothing was retransmitted or returned timestamp is less
  * than timestamp of the first retransmission.
  */
@@ -2700,9 +2671,8 @@ static bool tcp_try_undo_loss(struct sock *sk)
 	return false;
 }
 
-/* This function implements the PRR algorithm, specifcally the PRR-SSRB
- * (proportional rate reduction with slow start reduction bound) as described in
- * http://www.ietf.org/id/draft-mathis-tcpm-proportional-rate-reduction-01.txt.
+/* The cwnd reduction in CWR and Recovery use the PRR algorithm
+ * https://datatracker.ietf.org/doc/draft-ietf-tcpm-proportional-rate-reduction/
  * It computes the number of packets to send (sndcnt) based on packets newly
  * delivered:
  *   1) If the packets in flight is larger than ssthresh, PRR spreads the
@@ -2711,13 +2681,29 @@ static bool tcp_try_undo_loss(struct sock *sk)
  *	losses and/or application stalls), do not perform any further cwnd
  *	reductions, but instead slow start up to ssthresh.
  */
-static void tcp_update_cwnd_in_recovery(struct sock *sk, int newly_acked_sacked,
-					int fast_rexmit, int flag)
+static void tcp_init_cwnd_reduction(struct sock *sk, const bool set_ssthresh)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	tp->high_seq = tp->snd_nxt;
+	tp->bytes_acked = 0;
+	tp->snd_cwnd_cnt = 0;
+	tp->prior_cwnd = tp->snd_cwnd;
+	tp->prr_delivered = 0;
+	tp->prr_out = 0;
+	if (set_ssthresh)
+		tp->snd_ssthresh = inet_csk(sk)->icsk_ca_ops->ssthresh(sk);
+	TCP_ECN_queue_cwr(tp);
+}
+
+static void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked,
+			       int fast_rexmit)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	int sndcnt = 0;
 	int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);
 
+	tp->prr_delivered += newly_acked_sacked;
 	if (tcp_packets_in_flight(tp) > tp->snd_ssthresh) {
 		u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +
 			       tp->prior_cwnd - 1;
@@ -2732,43 +2718,29 @@ static void tcp_update_cwnd_in_recovery(struct sock *sk, int newly_acked_sacked,
 	tp->snd_cwnd = tcp_packets_in_flight(tp) + sndcnt;
 }
 
-static inline void tcp_complete_cwr(struct sock *sk)
+static inline void tcp_end_cwnd_reduction(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
-	/* Do not moderate cwnd if it's already undone in cwr or recovery. */
-	if (tp->undo_marker) {
-		if (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR) {
-			tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
-			tp->snd_cwnd_stamp = tcp_time_stamp;
-		} else if (tp->snd_ssthresh < TCP_INFINITE_SSTHRESH) {
-			/* PRR algorithm. */
-			tp->snd_cwnd = tp->snd_ssthresh;
-			tp->snd_cwnd_stamp = tcp_time_stamp;
-		}
+	/* Reset cwnd to ssthresh in CWR or Recovery (unless it's undone) */
+	if (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR ||
+	    (tp->undo_marker && tp->snd_ssthresh < TCP_INFINITE_SSTHRESH)) {
+		tp->snd_cwnd = tp->snd_ssthresh;
+		tp->snd_cwnd_stamp = tcp_time_stamp;
 	}
 	tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
 }
 
-/* Set slow start threshold and cwnd not falling to slow start */
+/* Enter CWR state. Disable cwnd undo since congestion is proven with ECN */
 void tcp_enter_cwr(struct sock *sk, const int set_ssthresh)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
-	const struct inet_connection_sock *icsk = inet_csk(sk);
 
 	tp->prior_ssthresh = 0;
 	tp->bytes_acked = 0;
-	if (icsk->icsk_ca_state < TCP_CA_CWR) {
+	if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
 		tp->undo_marker = 0;
-		if (set_ssthresh)
-			tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
-		tp->snd_cwnd = min(tp->snd_cwnd,
-				   tcp_packets_in_flight(tp) + 1U);
-		tp->snd_cwnd_cnt = 0;
-		tp->high_seq = tp->snd_nxt;
-		tp->snd_cwnd_stamp = tcp_time_stamp;
-		TCP_ECN_queue_cwr(tp);
-
+		tcp_init_cwnd_reduction(sk, set_ssthresh);
 		tcp_set_ca_state(sk, TCP_CA_CWR);
 	}
 }
@@ -2787,7 +2759,7 @@ static void tcp_try_keep_open(struct sock *sk)
 	}
 }
 
-static void tcp_try_to_open(struct sock *sk, int flag)
+static void tcp_try_to_open(struct sock *sk, int flag, int newly_acked_sacked)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
@@ -2804,7 +2776,7 @@ static void tcp_try_to_open(struct sock *sk, int flag)
 		if (inet_csk(sk)->icsk_ca_state != TCP_CA_Open)
 			tcp_moderate_cwnd(tp);
 	} else {
-		tcp_cwnd_down(sk, flag);
+		tcp_cwnd_reduction(sk, newly_acked_sacked, 0);
 	}
 }
 
@@ -2898,7 +2870,6 @@ static void tcp_enter_recovery(struct sock *sk, bool ece_ack)
 
 	NET_INC_STATS_BH(sock_net(sk), mib_idx);
 
-	tp->high_seq = tp->snd_nxt;
 	tp->prior_ssthresh = 0;
 	tp->undo_marker = tp->snd_una;
 	tp->undo_retrans = tp->retrans_out;
@@ -2906,15 +2877,8 @@ static void tcp_enter_recovery(struct sock *sk, bool ece_ack)
 	if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
 		if (!ece_ack)
 			tp->prior_ssthresh = tcp_current_ssthresh(sk);
-		tp->snd_ssthresh = inet_csk(sk)->icsk_ca_ops->ssthresh(sk);
-		TCP_ECN_queue_cwr(tp);
+		tcp_init_cwnd_reduction(sk, true);
 	}
-
-	tp->bytes_acked = 0;
-	tp->snd_cwnd_cnt = 0;
-	tp->prior_cwnd = tp->snd_cwnd;
-	tp->prr_delivered = 0;
-	tp->prr_out = 0;
 	tcp_set_ca_state(sk, TCP_CA_Recovery);
 }
 
@@ -2974,7 +2938,7 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
 			/* CWR is to be held something *above* high_seq
 			 * is ACKed for CWR bit to reach receiver. */
 			if (tp->snd_una != tp->high_seq) {
-				tcp_complete_cwr(sk);
+				tcp_end_cwnd_reduction(sk);
 				tcp_set_ca_state(sk, TCP_CA_Open);
 			}
 			break;
@@ -2984,7 +2948,7 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
 				tcp_reset_reno_sack(tp);
 			if (tcp_try_undo_recovery(sk))
 				return;
-			tcp_complete_cwr(sk);
+			tcp_end_cwnd_reduction(sk);
 			break;
 		}
 	}
@@ -3025,7 +2989,7 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
 			tcp_try_undo_dsack(sk);
 
 		if (!tcp_time_to_recover(sk, flag)) {
-			tcp_try_to_open(sk, flag);
+			tcp_try_to_open(sk, flag, newly_acked_sacked);
 			return;
 		}
 
@@ -3047,8 +3011,7 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
 
 	if (do_lost || (tcp_is_fack(tp) && tcp_head_timedout(sk)))
 		tcp_update_scoreboard(sk, fast_rexmit);
-	tp->prr_delivered += newly_acked_sacked;
-	tcp_update_cwnd_in_recovery(sk, newly_acked_sacked, fast_rexmit, flag);
+	tcp_cwnd_reduction(sk, newly_acked_sacked, fast_rexmit);
 	tcp_xmit_retransmit_queue(sk);
 }
 
@@ -3394,7 +3357,7 @@ static inline bool tcp_may_raise_cwnd(const struct sock *sk, const int flag)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 	return (!(flag & FLAG_ECE) || tp->snd_cwnd < tp->snd_ssthresh) &&
-		!((1 << inet_csk(sk)->icsk_ca_state) & (TCPF_CA_Recovery | TCPF_CA_CWR));
+		!tcp_in_cwnd_reduction(sk);
 }
 
 /* Check that window update is acceptable.
@@ -3462,9 +3425,9 @@ static void tcp_conservative_spur_to_response(struct tcp_sock *tp)
 }
 
 /* A conservative spurious RTO response algorithm: reduce cwnd using
- * rate halving and continue in congestion avoidance.
+ * PRR and continue in congestion avoidance.
  */
-static void tcp_ratehalving_spur_to_response(struct sock *sk)
+static void tcp_cwr_spur_to_response(struct sock *sk)
 {
 	tcp_enter_cwr(sk, 0);
 }
@@ -3472,7 +3435,7 @@ static void tcp_ratehalving_spur_to_response(struct sock *sk)
 static void tcp_undo_spur_to_response(struct sock *sk, int flag)
 {
 	if (flag & FLAG_ECE)
-		tcp_ratehalving_spur_to_response(sk);
+		tcp_cwr_spur_to_response(sk);
 	else
 		tcp_undo_cwr(sk, true);
 }
@@ -3579,7 +3542,7 @@ static bool tcp_process_frto(struct sock *sk, int flag)
 			tcp_conservative_spur_to_response(tp);
 			break;
 		default:
-			tcp_ratehalving_spur_to_response(sk);
+			tcp_cwr_spur_to_response(sk);
 			break;
 		}
 		tp->frto_counter = 0;
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 9383b51f3efc..cfe6ffe1c177 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2037,10 +2037,10 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 		if (push_one)
 			break;
 	}
-	if (inet_csk(sk)->icsk_ca_state == TCP_CA_Recovery)
-		tp->prr_out += sent_pkts;
 
 	if (likely(sent_pkts)) {
+		if (tcp_in_cwnd_reduction(sk))
+			tp->prr_out += sent_pkts;
 		tcp_cwnd_validate(sk);
 		return false;
 	}
@@ -2542,7 +2542,7 @@ begin_fwd:
 		}
 		NET_INC_STATS_BH(sock_net(sk), mib_idx);
 
-		if (inet_csk(sk)->icsk_ca_state == TCP_CA_Recovery)
+		if (tcp_in_cwnd_reduction(sk))
 			tp->prr_out += tcp_skb_pcount(skb);
 
 		if (skb == tcp_write_queue_head(sk))